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£S ECORD COPY ESO ACCESSION LIST. tSTI ___*t JLLLI -
Capy ^ /- * - . J . ■
2
Solid State Research 1967
pared under LI »n Contract Ar IVlo^HJOlo/ by
Lincoln Laboratory MASSACHUSETTS INSTITUTE OF TECHNOLOGY B
Lexington, Massachusetts
The work reported in this document was performed at Lincoln Laboratory, a center for research operated by Massaehus. ate of Technology, with the support of the U.S.Air Force under Contract AF 19(628)-5167.
This report may be reproduced to satisfy needs of U.S. Government ager,
nt is unlimir
Non-Lincoln Recipients
PLEASE DO NOT RETURN
Permission is given to destroy this document when it is no longer needed.
2
Solid State Research 1967
Issued 5 July 1967
Lincoln Laboratory MASSACHUSETTS INSTITUTE OF TECHNOLOGY
g Lexington, Massachusetts
ABSTRACT
This report covers in detail the solid state research work
at Lincoln Laboratory for the period 1 February through
30 April 1967. The topics covered are Solid State Device
Research, Optical Techniques and Devices, Materials Re-
search, and Physics of Solids.
Accepted for the Air Force Franklin C. Hudson Chief, Lincoln Laboratory Office
in
INTRODUCTION
I. SOLID STATE DEVICE RESEARCH
Single crystals of Pb. Sn Sehave been grown by the Bridgman technique with compositions
up to y = 0.3. The as-grown material exhibits a very high degree of Pb-Sn ratio homo-
geneity but is consistently heavily p-type with high carrier densities. Carrier concentra-
tions and 77°K Hall mobilities of 9 * 10 cm" and 25,000cm /V sec in p-type material, 17-3 2 and of 1.5 x 10 cm and 42,000cm /V sec in n-type material, have been obtained by an-
nealing the material in a two-zone furnace.
The interdiffusion parameters of Pb and Se in PbSe have been studied by introducing con-
trolled deviations from the stoichiometric concentration. Excess Se was diffused into
Pb-rich n-type material and excess Pb was diffused into Se-rich p-type material, forming
in each case a p-n junction. The diffusion constants were obtained by plotting the junction
depth as a function of time.
Pb, Sn Se lasers and photovoltaic detectors have been fabricated over a wide composition
range. Laser emission has now been observed out to 18.9^ and photovoltaic response to
22 \i at 12°K. Close agreement is observed between the emission wavelength and the cutoff
wavelength of the photovoltaic response in lasers and detectors fabricated from the same
material. Results show that the energy gap of Pb, Sn Se varies with Sn content in much
the same manner as that of Pb, Sn Te. 1-x x
Efficient photovoltaic detectors of infrared radiation at wavelengths up to 1 l\x which oper-
ate at 77°K have been fabricated from annealed Bridgman-grown Pb. Sn Se single crys-
tals. Responsivities of 3 V/W at 77°K, and internal quantum efficiencies of about 20 per-
cent, have been obtained. The detector noise is less than the noise of the measuring sys-
tem yielding an upper noise limit of 10" V sec. This corresponds to a minimum D* of 9 1/2
3 x 10 cm/W sec ' near the peak at 11 p.. The detector risetime at 77°K is 20nsec which
can be accounted for by the RC time constant of the device.
A method has been proposed by which infrared radiation is efficiently converted directly to
visible radiation by a single solid state device. The device consists of a capacitor-
photodetector-photoemitter sandwich in which infrared radiation is incident on one face
and the visible radiation is emitted from the opposite face. The low level current produced
by the infrared detector is integrated and stored by the capacitor and then delivered to the
light emitter in short high current pulses. The feasibility of such a pulsed device has been
demonstrated using an indium antimonide diode detector and a gallium arsenide-phosphide
diode emitter to convert infrared radiation of wavelengths up to 5.3 \x into visible ra< -4
between 0.6 and 0.7 JJL with a quantum efficiency of 10 .
Electroluminescence has been observed at 4.2°and77°K from CdS vapor-grown single crys-
tal platelets with indium contacts bonded to opposite faces of the 10- to 20-n thick samples.
Introduction
4 -2 -2 Current densities as high as 2 * 10 Acm under pulsed conditions and 100Acm DC _2
have been obtained. At low temperatures, luminescence appears at about 10 Acm DC
with light intensity increasing rapidly with current. External quantum efficiencies between
10"6 and 10"5 were obtained at 103 Acm"2.
We have obtained laser emission from ZnTe excited by an electron beam with up to 90 W
of peak output power at 5280A and 8-percent overall power efficiency at liquid helium o
temperature, and up to 25 W at 5310 A and 2-percent overall power efficiency at 77°K.
The lowest threshold observed was 48keV, which is high compared with the other Il-Vl
compounds.
II. OPTICAL TECHNIQUES AND DEVICES
A CW Doppler radar has been constructed using a 5-W CO« laser and a helium-cooled Cu-
doped Ge heterodyne detector. Moving vehicles have been detected out to ranges of 2 miles.
A sealed-off 5-W CO«, laser tube has been operated for more than 700 hours by reclaim-
ing the CO„ gas from the cathode region. The C02 adsorption at the nickel cathode was
reversible, and this gas was again replaced in the tube fill by heating the cathode region
to 300 °C.
Narrow-band operation of a Pb Sn Se photovoltaic detector at 77°K has yielded hetero-
dyne detection sensitivities close to the theoretical limit.
Faraday rotation in indium antimonide is being used to construct an isolator at 10.6-u-m
wavelength.
III. MATERIALS RESEARCH
Single crystals of MnBr2 have been grown from the melt and from the vapor phase by a
method in which helium was used as a carrier gas to transport MnBr2 vapor produced by
the reaction between Br? vapor and Mn powder at 850°C. A large-grained ingot of EuO
was prepared in a sealed molybdenum crucible by solidification of a nonstoichiometric
melt heated to 2000°C and slowly cooled.
Isothermal measurements of resistivity as a function of time for S-doped GaSb have shown
that the transfer of electrons from the conduction band to the sulfur donor levels is char-
acterized by two time constants, both of which vary exponentially with reciprocal temper-
ature. The slower constant increases from 5 min. at 90°K to 1300 min. at 77°K, and the
faster constant increases from 1.6 to 230 min. over the same temperature range.
Data on the resistivity of Ag2Te as a function of temperature and hydrostatic pressure
have been used to construct a pressure-temperature diagram for this compound. The
structure of Ag2Te-III, which is the stable phase at room temperature for pressures above
25 kbars, was found to be tetragonal on the basis of x-ray diffraction results obtained with
a new high-pressure camera.
vi
Introduction
The effect of high pressure on the properties of titanium monoxide (TiO ), which has the
rocksalt structure, has been studied by annealing samples with x between 0.87 and 1.06
at about 50kbars and 1100° to 1300°C for 1 to 3 hours. This treatment reduced the con-
centration of vacancies from 14 - 15 to 11 - 13 percent of the total number of lattice sites,
and increased the superconducting transition temperature from less than 1.2°K to be-
tween 1.33 and 1.8°K.
Measurements with a vibrating coil magnetometer have shown that the Curie temperature
of ferromagnetic SrRuO„, which is 164°K at atmospheric pressure, decreases linearly
with increasing hydrostatic pressure at the rate of 0.63°/kbar. This decrease supports
the hypothesis that the magnetic properties of this material are primarily due to band ef-
fects rather than to localized moments.
An electron density difference map for PbRuO- has been obtained by computer calculation
from integrated x-ray intensity data. This map confirms the existence of a trap-mediated
Pb-Pb bond in PbRuO«, as proposed previously on the basis of structure refinement
calculations.
An analysis has been made of the factors influencing the Neel temperature in magnetic
materials with perovskite and related structures. It was shown that the theoretical ex-
pression for the variation of Neel temperature with lattice parameter cannot be adequately
tested if the lattice parameter is changed by chemical substitution rather than by applying
pressure.
IV. PHYSICS OF SOLIDS
In extending the magnetoelectroreflection studies, a mechanically integrated thin film
package has been developed. This dry package has eliminated the temperature and wave-
length restrictions which have limited the liquid electrolyte technique, thereby making the
method competitive with piezoreflectance.
The anomalous exciton peaks previously reported for InSb have not been observed in ger-
manium. This result, taken in conjunction with calculations by Bell and Rogers, suggests
that the anomalies arise from the lack of inversion symmetry of InSb.
A magneto-optical investigation is being undertaken to determine energy band changes in
the transition from semimetal to semiconductor as bismuth is alloyed with antimony. To
date, sharp magnetoreflection oscillations have been observed in the alloy Bifl a-fib^ Q3.
The current modulated reflectance data of gold has been analyzed. Qualitative agreement
with the observed structure was obtained.
Recent experimental investigations of the magnetic energy levels of donor impurities in
InSb have prompted a variational calculation of the impurity ground state and the lowest
lying M = ±1 excited states. Comparison of the results with experiment suggests tenta-
tively a central cell correction which increases with magnetic field.
vii
Introduction
Work on the phonon dispersion relations for silicon and germanium is continuing. By in-
corporating the shell model within the Fourier expansion technique, a more rapid con-
vergence of the series is obtained; for silicon, calculations using up to and including
fourth-nearest-neighbor terms for the ion cores, and the lowest nonvanishing terms for
the core shell interaction, give reasonable agreement with the experimental neutron dif-
fraction data.
In order to explain the transport anomalies in Ti2
Introduction
is consistently 20 percent closer than expected from the cavity lengths, is likely to be
caused by quartz resonator mode pulling.
A technique has been developed for producing high-power pulses of coherent light with fre-
quency continuously tunable over a lOOO-A range. Although so far the method has been
used to generate visible light, in principle it is applicable for a tunable far infrared source.
High power mode locked output has been obtained in a rotating prism Q-switched ruby
laser without the use of separate cavity modulators. The number of modes involved is
small and the deduced peak power is 50 MW.
A cyanide laser, which operates at 337fi. has been constructed. It will be used for far in-
frared spectroscopic investigations.
CONTENTS
Abstract iii Introduction v Organization xii Reports by Authors Engaged in Solid State Research xiii
I. SOLID STATE RESEARCH 1
A. Crystal Growth, Composition Control and Junction Formation in the Lead-Tin Chalcogenides 1
B. Diffusion Studies in Lead Salts 4
C. Bandgap Determination from Laser Emission and Photovoltaic
Response Measurements in Pb, Sn Se Diodes 6
D. Properties of Pb. Sn Se Infrared Detectors 7
E. InSb-GaAsP Infrared to Visible Light Converter 9
F. Electroluminescence in CdS 11
G. Laser Emission from Electron-Beam-Excited ZnTe 11
II. OPTICAL TECHNIQUES AND DEVICES 15
A. C02 Laser Radar System 15
B. Sealed C02 Laser Tube with Greater than a 700-Hour Life 16
C. Photovoltaic Heterodyne Detection in Pb Sn Se 17
D. Faraday Isolator 18
III. MATERIALS RESEARCH 19
A. Crystal Growth 19
B. Kinetics of Electron Transfer Between Conduction Band and Sulfur Donors in GaSb 19
C. Polymorphism in Ag2Te at High Pressures and Temperatures 21
D. Effect of High Pressure on Oxides with Defect-Rocksalt Structures 24
E. Effect of Hydrostatic Pressure on the Curie Point of SrRu03 25
F. Election Density Difference Map as Further Evidence for Trap- Mediated Pb-Pb Bond in PbRu03 26
G. Neel Temperatures in Perovskite and Related Structures 27
IV. PHYSICS OF SOLIDS 31
A. Electronic Band Structure 31
B. Transport Phenomena 3 5
C. Magnetism 37
D. Scattering Experiments with Lasers 45
ORGANIZATION
.SOLID STAT1 DIVISION
V I.. Mt Shorter. Head E. TaaacftwaM, Associate ffead
M. J. Hudson. Issistant E. P. Varekois
SOLID STATE TIIEOItt
II. J. /.eiger. Leader \l. M. I.itvak, Assistant Leader
Argyres, P. N. Chinn, S. S Dresselhaus. G. K. H.IIM1S. J.
Kaplan. T. A. Kellev. P. I . Kleiner. R. II.
Larsen, D. M.
Melanson. (. V. |r Palm. B. I.* Stanley, II. E. Trent/P. II. \.in /andt. I.. I..
(»I'll«- WD IM -HMD-I)
R. II. Kingston. Leader H.J. Kcyes, Assistant Leader
Dates. I). II. H.M.a. ii. A. Carbone. R. J. Dennis. J. II. Freed. C. Longakcr. P. R.
UcPhie. | \l. n.nsi, I. \l. Ross, A. II. \|. Sullivan. I . \l Teicfc, M. C. /immerman. \l D.
I l I CTRONK MATEBIALS
J. B. Goodenough. Leader J. \l. Honig, Associate Leader A. J. Strauss, Assistant Leader
Anderson, C. II.. Jr. Andrews. H. 1.1 Arnolt, R. J. Banus, M. D. Batson, D. Brebrick. R. I .. It. Button, M. J. Delanev. I I England, R. E. Kahey, R. E. Ferrctti, A. Finn. M. C. Hilsenrath, S. Iseler, C. R. Kafalas, J. A.
LaFleur. R. J. Eavine, M. C. Longo, J. \l. Mastromattei. I . I O'Connor, J. R. Owens, E. B. Plonko. M. C. Pollard. E. R.t Raccah. I\ M Reed, T. B. Roddy. J. T. Searles, I. H. Sohn. J. B. Soracco, D. J. Steininger, J. A.
SOLID STATI PHYSICS
J. G. \Ia\r..ides, Leader G. B. Rright, Assistant Leader
Burke, J. R. Carman, R. L. Dickey, D. H. Dresselhaus. M. s
Dwight. K.. Jr. F'einleib, J. Feldman. B. Fulton. M. J. Groves. S. H. Johnson, E. J. Kernan, R. C. Kolesar. D. F. Krag. R. E.
Vtelngailis, J. Menyuk, N. Murphv. II. C N.ll. K. R. Parker, C. D. Perry, F. H. Scouler, R. J. Stickler, J. J.t Strahm. N. D.t Thaxter, J. B. Tichovolsky. E. J.t Reber, R.
\IMM II I) PHYSICS
J. 0. Dimmock, Leader I I llarman. Assistant Leader I. Mclngailis, Assistant Leader
Butler, I. I . Calawa. \ H. Carter. I B. Caswell. F. 11. Clough, T. F. Donaldson, P. L. Donnelly. J. P. Finne. P. Foyt, A. G. Hinkley, E. D.
Hurwitz, C. E. Lindley. R. I Mooradian, A. Oliver, M R.1 Paladino, A. E. Phelan. R. J.. Jr. Rard. J. II. R.. Ill Wolfe, C. M. Youtz, P.
* Part Time
t Research Assistant
XII
REPORTS BY AUTHORS ENGAGED IN SOLID STATE RESEARCH
15 February through 15 May 1967
PUBLISHED REPORTS
JA No.
2581 Magnetic Quantum Effects
Journal Articles*
P.N. Argyres L.M. RothT
Semiconductors and Semimetals. Vol. 1, Physics of I1I-V Com- pounds, edited by R.K. Willardson and A.C. Beer (Academic Press, New York, 1966)
2747 Space-Time Symmetry Restric- tions on Transport Coefficients. II. Two Theories Compared
W.H. Kleiner Phys. Rev. 153, 726(1967)
2796 Comparison of the Theoretical P.M.Raccah 0"~ Form Factors with R.J. Arnott Experiment
Phys. Rev. 153, 1028 (1967) DDC 651719
2837 Low Temperature Crystallo- N. Menyuk graphic and Magnetic Study of K. Dwight LaCoO, P.M.Raccah
J. Phys. Chem. Solids 28, 549 (1967)
2872 A First-Order Localized- Electron ^Collective-Electron Transition
P.M. Raccah J.B. Goodenough
Phys. Rev. 155, 932 (1967)
2873 Hartrec-Fock Theory: Slater T.A.Kaplan Determinants of Minimum W. H. Kleiner Energy
Phys. Rev. 156. 1 (1967)
2875 Virial Theorem for the Homo- P.N. Argyres geneous Electron Gas
Phys. Rev. 154, 410 (1967)
2878 Low Temperature Absolute Re- J. Feinleib flection Measurements of Small B. Feldman Samples
Rev. Sei. Instr. 38, 32 (1967)
2887 Spark-Source Mass Spectroscopy E.B.Owens
2890 Interband Magnetooptical Studies B. Laxt of Semiconductors and Semi- J.G. Mavroides metals
Appl. Spectroscopy 21, 1(1967)
Appl. Optics 6, 647(1967)
' Rcsrfaia available.
1 Author not at Lincoln Laboratory.
xiii
Reports
JA No.
2903 Excitation Spectra of Group III D.H. Dickey Impurities in Germanium Under J.O. Dimmock Uniaxial Stress
J. Phys. Chem. Solids 28, 529 (1967)
2918 Semiconductor-to-Metal Tran- sitions in Transition-Metal Compounds
D. Adler* J. Feinleib H. Brooks* W. Paul*
Phys. Rev. 155. 851 (1967)
2920
2921
Semiconductor-to-Metal Tran- J. Feinleib sition in V203 W. Paul*
On the Thermodynamic Proper- A.K. Jena* ties of Several Solid Phases of M. B. Bever* the Compound InSb M.D. Banus
Phys. Rev. 155, 841 (1967)
Trans. Met. Soc. 239, 725 (1967)
2923 Diode Lasers of Pb, Sn Se and Pb.-xSnxTe " "
J.F. Butler A.R. Calawa T.C. Harman
Appl. Phys. Letters 9, 427 (1966)
2927 Polar on Effects in the Cyclotron- Resonance Absorption of InSb
D.H. Dickey E.J. Johnson D.M. Larsen
Phys. Rev. Letters 18, 599 (1967)
2928 Temperature-Modulated Reflect- W.J.Scouler ance of Gold from 2 to 10 eV
Phys. Rev. Letters 18, 445 (1967)
2932 High Power and Efficiency in CdS Electron Beam Pumped Lasers
C.E. Hurwitz Appl. Phys. Letters 9, 420 (1966)
2946 InSb MOS Infrared Detector
2948 The Electrical Properties and Band Structure of Doped LalnCL
R.J.Phelan, Jr. J.O. Dimmock
D.B. Rogers* J.M. Honig J.B. Goodenough
Appl. Phys. Letters W, 55 (1967)
Materials Res. Bull. 2, 223 (1967)
2965 Identification of the Fundamental Vibrational Modes of Trigonal, Qf-Monoclinic and Amorphous Selenium
G. Lucovsky* A. Mooradian W. Taylor* G.B. Wright R.C. Keezer*
Solid State Commun. 5, 113 (1967)
2988
MS No.
1422
Raman Scattering from Donor and Acceptor Impurities in Silicon
Theory of the Method of Kinetic Equations for Quantum Systems
G.B. Wright A. Mooradian
P.N. Argyres
Phys. Rev. Letters 18, 608 (1967)
Lectures in Theoretical Physics, V0I.8A (University of Colorado Press, 1966)
1702 Optical and Laser Properties of Nd+3- and Eu+3-Doped YV04
J.R. O'Connor Trans. Met. Soc. 239, 362 (1967)
* \uthor not at Lincoln Laboratory.
Reports
MS No.
1746 Dependence of Critical Properties H. E. Stanley of Heisenberg Magnets upon Spin T. A. Kaplan and Lattice
J. Appl.Phys.38, 977 (1967)
1747 On the Possible Phase Transition H. E. Stanley for Two-Dimensional Heisenberg T.A. Kaplan Models
J. Appl.Phys.38, 975 (1967)
1791
1814-11
Spontaneous Band Magnetism J. B. Goodenough
Characterization of d Electrons in J.B. Goodenough Solids by Structure. II. Spontan- eous Crystallographic Distortions
J. Appl.Phys. 38, 1054 (1967)
Materials Res. Bull. 2, 165 (1967)
1815 Experimental Techniques with General Applicability for the Study of Magnetic Phenomena
K. Dwight J. Appl. Phys. 38. 1505(1967)
UNPUBLISHED REPORTS
JA No.
2850
Journal Articles
High-Temperature Expansions for the Classical Heisenberg Model. I. Spin Correlation Function
H.E. Stanley Accepted by Phys. Rev.
2907 High-Temperature Expansions for the Classical Heisenberg Model. II. Zero-Field Susceptibility
H.E. Stanley Accepted by Phys. Rev.
2939 Fourier Expansion for the Elec- tronic Energy Bands in Silicon and Germanium
G.F. Dresselhaus M.S. Dresselhaus
Accepted by Phys. Rev.
2964 Temperature Dependence, Orien- tation Correlation and Molecular Fields in Second Harmonic Light Scattering from Liquids and Gases
D.L. Weinberg Accepted by J. Chem. Phys.
2973 The Crystal Structure of Neodym- ium Monotellurooxide-NdjO Te
P.M. Raccah J.M. Longo H.A. Eick*
Accepted by Inorg. Chem.
2975 Report on the Seventh Interna- B. Lax* tional Conference on the Physics J.G. Mavroides of Semiconductors
Accepted by Phys. Today
2977 Interband Magnetoreflection and Band Structure of HgTe
2978 Magnetoplasma Cyclotron Absorption in PbSe
S.H. Groves R.N. Brown* C.R.Pidgeon*
S. Bermon
Accepted by Phys. Rev.
Accepted by Phys. Rev.
•Author not at Lincoln Laboratory.
XV
Reports
JA No.
2989 A Thermodynamic Investigation of the Compounds In»SbTe?, InSb and InTe 6 *
A.K.Jena* M.B. Bever* M.D. Banus
2993 Magnetic and Structural Study of the Spinel MnYhuS.
M.J. Longo P.M. Raccah
2994 Stability Measurements of C02-N2-He Lasers at 10.6|±m Wavelength
C. Freed
2998 Hall Coefficient and Transverse T.C. Harman Magnetoresistance in HgTe at 4.2°Kand 77°K
J.M. Honig P. Trent
3018 Growth of Single TuO Crystals from the Melt J
T.B. Reed R.E. Fahey J.M. Honig
Accepted by Trans. Met. Soc. AIME
Accepted by Materials Res. Bull.
Accepted by IEEE J. Quant. Electron.
Accepted by J. Phys. Chem. Solids
Accepted by Materials Res. Bull.
3023 Laser Emission from Electron C.E. Hurwitz Beam Excited ZnTe
Accepted by IEEE J. Quant. Electron.
MS No.
1629B Spontaneous Band Ferromag- netism in Metallic Oxides
Meeting Speeches t
J.B. Goodenough Materials Research Unit Seminar, McMaster University, Ontario, Canada, 20 March 1967
1629C Relation Between Structural and Magnetic Properties of Transition-Metal Compounds
1836 Effect of High Pressures on the Structures and Properties of Some Solids
J.B. Goodenough
M.D. Banus
Solid State Chemistry Conference, Brown University, 3-5 April 1967
1719E Interstellar OH Maser Emission A. L. McWhorter Seminar, University of Colorado, 17 March 1967
1787B Spin Waves in Paramagnetic Fermi Gases
L.L. Van Zandt Seminar, Wayne State University, Detroit, Michigan, 16 February 1967
1793A Polar on Self-Energy Effects in E.J.Johnson InSb
Seminar, Purdue University, 7 March 1967
1830B Metallic Properties of Certain J.M. Honig Metal Oxides
Colloquium, Brandeis University, 7 March 1967
* Audi or not at Lincoln Laboratory.
t Titles of Meeting Speeches are listed for information only. No copies are available for distribution.
XVi
MS No.
Reports
1846A.B An Effective Ikimiltonian lor the Optical Properties of Si and Ge
G. F. Dresselhaus Seminar, New York Univcr .'4 February 1967; Colloquim. University of Illinois. 17 March 1967
1848 Kinetics of Electron Transfer Between Conduction Band Bad Sulfur Donors in GaSb
1860 Fourier Expansion of the Phonon Dispersion Relations for Silicon and Germanium
1861 Magnetooptical Evidence for the Existence of a Complex Band Edge at the Energy Gap of the Layer Compound GaSe
1 B66 Current Modulated Reflectance of Nickel from 0.1 to lOeV
1867 Observation of Exctton Fine Structure in the Interband Magneto-Atsorption of InSb
1868 Magnetoelectroreflectance at 1.5°K
1869 NMR in the Ferromagnetic Semiconductor CdCr^Se, 2 4
1873 Critical Behavior of the Zero- Field Susceptibility for Heisenberg Models
1874 Negative Magnetoresistance Effects in Ti^
1875 Symmetry of the Ground Level of a Hamiltonian
1876 High Efficiency Infrared Photo- voltaic Effect in Metal-Oxide- Semiconductor Structures
1877 Observations on the Existence of a Phase Transition in Two- Dimensional Heisenberg Models
G.W. Iseler AJ. Strauss
G. F. Dresselhaus
J. Halpern J.L. Brebner* E. Mooser*
W.J. Scouler ]. Feinleib J. Hanus
E.J. Johnson
C.R. Pidgeon* J. Feinleib S.H. Groves R.J. Phelan, Jr.
M. Rubinstein* G.H. Stauss* J. Feinleib A. Wold*
H.E. Stanley
J. M. Honig L. L. Van Zandt J.B. Sohn T.B. Reed
W.H. Kleiner T.A. Kaplan
R.J. Phelan, Jr. J.O. Dimmock
T.A. Kaplan H.E. Stanley
American Physical Society, Chicago, 27-30 March 1967
* \uthor nol at Lincoln Laboratory.
xvii
Reports
MS No.
1882
1891B
1924
1931
1850
Landau Damping of Magneto- plasma Waves for General Closed Fermi Surfaces
Laser Action and Photovoltaic Effect in Pb, Sn Se Diodes
1-y y
Raman Scattering from Plasmons and Phonons
Inversion of Conduction and Valence Bands in Pb. Sn Se Alloys I_X X
Microwave Magnetoacoustic Effects in Indium Antimonide
Optical Modulation in Semi- conductors and Metals
Recent Developments in Mass Spectroscopy
A.L. McWhorter J.N. Walpole*
J. F. Butler A.R. Calawa I. Melngailis T.C. Harman J.O. Dimmock
A. Mooradian
A.J. Strauss
K. W. Nill
J. Feinleib
E.B. Owens
American Physical Society, Chicago, 27 - 30 March 1967
Society of Applied Spectroscopy, Newton, Massachusetts, 10 January 1967
1882B Raman Scattering in Semiconductors
A. Mooradian Seminar, Purdue University, 7 April 1967
1882C Raman Scattering from Plasmons G.B.Wright and Phonons
Seminar, M.I.T. , 11 April 1967
1891 Physical Properties and Band Structure of Pb, Sn Se Alloys
A.J. Strauss Colloquium, Naval Ordnance Laboratory, Silver Spring, Maryland, 21 February 1967
1899 Properties of Pb!_xSnxTe and Pb, Sn Se Infrared Detectors
1907 Physical Properties and Band Structure of Pbi_xSnxTe and Pbi-xSnxSe A11°ys
1908 Infrared Heterodyne Detection of CO2 Laser Radiation in Ge:Cu and in Pb Sn Te
1903 Light-by-Light Scattering and Pulse Self-Steepening
I. Melngailis
A.J. Strauss T.C. Harman
M.C.Teich
P.L.Kelley
IRIS Infrared Detector Specialty Group, Wright-Patterson Air Force Base, 6- 7 Majch 1967
Seminar, Ford Motor Scientific Research Laboratory, Dearborn, Michigan, 13 February 1967
* Author not at Lincoln Laboratory.
xviii
Reports
MS No.
1904 High-Temperature Expansion of the Spin Correlation Function for Heisenberg Models
H.E. Stanley Seminar, Bell Telephone Laboratories, Murray Hill, New Jersey, 16 February 1967
1915 The Heisenberg Model of H.E.Stanley Magnetism
1919 Band Structure and Applications I. Melngailis ofPb, Sn Te and Pb, Sn Se 1-x x 1-y y
193 IB, C Optical Modulation in Sem icon- J. Feinleib duccors and Metals
Colloquium, Temple University, 6 March 1967
Seminar, Colorado State University, 2 March 1967
Seminar, Harvard University, 26 April 1967; Seminar, Ford Motor Scientific Research Laboratories, Dearborn, Michigan, 8 May 1967
1952A Observation of Magneto-Elastic R.Weber Coupling by Spin Wave Resonance
Seminar, Case Institute, Cleveland, Ohio, 9 May 1967
1964 On High-Temperature Expansions H.E. Stanley for the Heisenberg Model
17th Statistical Mechanics Meeting, Yeshiva University, New York, 6 April 1967
1968 Negative Magnetoresistance
1975
Effects in TiJD*
A Carbon Dioxide Laser Radar System
L.L. Van Zandt
H.A. Bostick
Seminar, Purdue University, 25 April 1967
3rd DoD Conference on Laser Technology, Pensacola, Florida, 18 - 20 April 1967
1977 Polarons in Indium Antimonide D.M. Larsen Seminar, Naval Research Laboratory, Washington, D.C., 21 AprU 1967
2001 Dynamics of Intense Light Beams P.L. Kelley in Nonlinear Media
Colloquium, Pennsylvania State University, 9 May 1967
xix
I. SOLID STATE DEVICE RESEARCH
A. CRYSTAL GROWTH, COMPOSITION CONTROL AND JUNCTION FORMATION IN THE LEAD-TIN CHALCOGENIDES
1. Bridgman Crystal Growth of Pbi-ySnySe
Single crystals of lead-tin selenide have been grown by the Bridgman technique and have
exhibited the same high degree of Pb-Sn ratio homogeneity as reported for Pb. Sn Te last
quarter. The alloy liquid compositions and the corresponding first-to-freeze solid composi-
tions are given in Table 1-1 which shows that the Pb-Sn ratio in the solid phase is approxi- mately 0.5 times that of the liquid. The Hall coefficient and electrical resistivity of the crys-
tals cut from the first-to-freeze section of the ingots were measured at 77°K. The carrier
concentration and carrier mobility were determined from the results of the electrical measure-
ments. Table 1-1 shows that the as-grown crystals were p-type with high hole-carrier densities.
Drawing an analogy between these and other lead salt crystals leads to the belief that the excess Se concentration of these Bridgman-grown crystals is greater than the hole-carrier density due
to the presence of electrically neutral Se microprecipitates. In order to obtain device grade material, the crystals must be annealed under rather stringent conditions.
2. Isothermal Annealing of Pbi_xSnxTe and Pbi_vSnvSe
Previous studies on the isothermal annealing of Pb. Sn Te were carried out on single
crystal specimens of 1-mm thickness, for annealing times up to 293 hours, and temperatures
TABLE 1-1
LIQUID COMPOSITION, SOLID COMPOSITION AND CARRIER CONCENTRATION OF BRIDGMAN-GROWN CRYSTALS OF Pb, Sn Se
1-y y
Ingot No.
Liquid Composition
y
First-to-Freeze Solid Composition
y
First-to-Freeze Hole- Corrier Concentration
at77°K (cm-3)
Hall Mobility at 77°K
(cm2/V sec)
67-4 0.12 0.064 19
3.9x 10 220
66-3 0.13 0.074 19
1.8X 101 1850
67-5 0.14 0.078 19
2.2X10 920
67-6 0.16 0.088 4.2x 1019 629
67-7 0.18 0.095 5.2 X1018 795
67-8 0.40 0.241 19
3.2X101 3820
67-3 0.50 0.308 19
7.0X10 260
Section I
TABLE 1-2
RESULTS OF ISOTHERMAL ANNEALING EXPERIMENTS OBTAINED ON BRIDGMAN-GROWN CRYSTALS
Composition Pb.. «Sn- ~\e (metal-rich only)
Carrier Carrier Sample Sample Annealing Concentration Mobility
Temperature (°C)
Thickness (mm)
Time (hr)
at 77°K (cm-3)
at 77°K (cm2/V sec)
750 0.25 21 - -
650 0.25 additional 43 1 Q
2.2 x 10 p-type
1,350
600 0.25 additional 912 4.3* 1017 1,730
450 0.25 additional 168 1.6 X 10 n-type 15,000
Composition Pb0935Sn0()65Se
Equilibrated with metal-rich powder
750 1.0 72 7.2 X1018 11,600
650 0.9 120 4.1 X1018 n-type
19,300
550 0.26 96 1.8 X1018 18,700
400 - - 3.36 X1017 26,000
Equilibrated with Se-rich powder
750 1.0 72 19
3 X10 -
650 1.0 168 19
3.8X 10
19 1.3X10
p-type -
550 0.43 312 3,100
400 0.012 24 7.1 X1018. 2,300
Section I
of 650°c to 825°e. in this quarter, 1',,1 _NSri
x I •• annealin riments were carried out with
luced samplr thickness of 0.25 trim, Longer annealing times and lower annealing tcmperatui
ihown in the upp blc 1-2, low eoneentration p-t n-type crystals with
X = 0.20 were achieved. [h«se results suggest that the solidus In,. is in equilibrium with
the metal-rich material intersects the stoichiometric composition line at about 46S*C for x - 0.20.
The isothermal annealing expert mi .'ended to the I * i >, -tem. !-;•
I in the lower section of Table 1-2 which shows that high concentration
material is obtained by mal annealing. The i ample annealed at
400°C for 84 hours was p iv metal-saturated at a higher tcmperatui | ). Thus, metal
p-type
25,000
20,000
T.C. Harman A. R. Calawa Mary C. Finn
Section I
B. DIFFUSION STUDIES IN LEAD SALTS
A study of the interdiffusion parameters of Pb and Se in PbSe has been initiated. The tech-
nique employed consists of introducing controlled deviations from the stoichiometric concentra-
tion of Pb and Se in PbSe. Excess Se in the material makes it p-type while excess Pb makes
it n-type. In these experiments excess Se was diffused into n-type and excess Pb into p-type
PbSe forming in each case a p-n junction. The material is assumed to be stoichiometric at the
junction and the junction depth was measured as a function of diffusion time and temperature.
If the diffusion coefficient is independent of Pb and Se concentration the solution of the dif-
fusion equation can be written as
c-co-ics-co)(i-eri-^J (I,
where C is the excess concentration of the diffusion species in the sample and C is the initial
excess concentration in the bulk sample. In the present experiments the diffusion species is
actually deficient in the bulk sample, such that C is a negative number taken to be equal in
magnitude to the deficiency in the bulk sample. C is the surface excess concentration of the
diffused species for the given experimental conditions, x is the distance into the sample, D
the diffusion coefficient and t the diffusion time. Since the excess concentration C is taken
to be zero at the junction, the diffusion constant D is obtained, once C and C are known, by s o
plotting the square of the junction depth as a function of the diffusion time.
The experiment consists of placing a p- or n-type sample of known concentration CQ into
an evacuated quartz ampule with a Pb- or Se-rich two-phase ingot and heating isothermally. With a two-phase ingot which contains sufficient excess Pb or Se, the surface excess concentra- tion C established through the vapor is equal to the saturation concentration at the liquidus- solidus boundary of PbSe at the given temperature. In order to obtain these saturation concen-
trations at the boundary of the solidus field small samples were equilibrated with an appropriate
excess two-phase ingot and quenched to room temperature. The saturation concentrations are
shown as a function of temperature in Fig. 1-1. These were determined by measuring the car-
rier concentration in n- and p-type samples. This will give the excess concentration of Pb or
Se, considering that each excess Se atom in a lattice site contributes one hole and each excess
Pb atom contributes one electron, provided the sample contains no precipitates. The Pb saturation values obtained in the present work agree quite well with published
values; however, the Se saturation values are considerably higher. The latter suggests that 4
considerable precipitation of the Se out of the lattice sites may occur during the cooling process. On the Se side, a temperature is reached above which the samples can no longer be quenched
without precipitation. This temperature, about 650°C for these PbSe samples, is reached when
the sample cooling rate is comparable to the precipitation rate. The maximum cooling rate is
limited by the sample characteristics such as specific heat, thermal conductivity and sample thickness. The thickness of the samples used in these experiments was 2.5 X 10" cm.
Th« initial concentration C is equal to the total excess of Pb or Se in the sample including
any precipitates. Since there is no direct method of obtaining the precipitate concentration, it
is necessary to be certain that the sample contains no precipitates and that the Hall concentration
Section I
'0~P
f '0
IS-SS-HM
O Pb-SATURATED SAMPLES
6 St-SATURATED SAMPLES
IS-SS-SH«]
O DIFFUSION INTO p-TYPE PbSt FROM A Pb-RICH SOURCE
A DIFFUSION INTO n-TYPE PbS« FROM A St-RICH SOURCE
J .
IO5/T
Section I
C. BANDGAP DETERMINATION FROM LASER EMISSION AND PHOTOVOLTAIC RESPONSE MEASUREMENTS IN Pfc^ Sn Se DIODES
Pb Sn Se lasers and photovoltaic detectors have been fabricated over a wide composition
range. Laser action has now been observed out to 18. V and photovoltaic response to 22 \i, both
at 12°K. Results show that the energy gap of Pbj Sn Se varies with Sn content in much the same manner as that of Pb SnxTe. Lasers and photovoltaic detectors have previously been
reported for several compositions of Pb^^n^e (Refs. 6-8) and a laser for one composition
ofPb SnySe8 g
Fabrication methods for all of the lasers were essentially the same as previously described.
The lasers exhibit spontaneous emission peaks 2 to 3 meV wide at 12°K. For currents above
threshold, characteristic mode structure is observed, with line widths less than 10 eV. The detectors were fabricated in approximately the same manner as those described in the following
section (I-D). No attempt was made to optimize their detectivity. Positions of emission peaks for P^ Sn Se lasers and long wavelength cut-off energies for
detectors have been plotted against Sn concentration in Fig. 1-3. The abscissa is taken only to
y = 0.4 since it is believed that the crystal structure undergoes a transformation at about this
concentration. A close agreement is evident between the wavelength of emission and the cut- off wavelength of the photovoltaic response. Hence, the photovoltaic cut-off appears to cor-
respond closely to the energy gap, as was proposed previously for the wavelength of laser
> —
I i jy mm
LUMINESCENCE (12*K)
LUMINESCENCE (77'K)
PHOTOVOLTAIC RESPONSE (I2»K)
PHOTOVOLTAIC RESPONSE
Section I
emission. Note that the temperature dependence of the energy gap of the PbQ 7SnQ «Se detector
is opposite to that of the devices with lower Sn concentration. This reversal is in agreement
with a model for the band structure of the Pb Sn Se alloys in which the valence and conduction J J
bands cross over at an intermediate composition as was first proposed for Pb. Sn Te. From
Fig. 1-3 the cross over in Pb. Sn Se appears to occur at y = 0.15 at 12°K and at y = 0.18 at 77°K.
For Pb. Sn Te the crossover occurs at about x = 0.33 at 12°K and at about 0.37 at 77°K. It was 1-x x proposed that the composition dependence of the energy gap in Pb. Sn Te was due largely to the difference in the relativistic corrections for Pb and Sn. If this model is applied to the selenide
it predicts a composition dependence of the energy gap in good qualitative agreement with
experiment. , p BuUer J. O. Dimmock T. C. Harman
D. PROPERTIES OF Pbj Si^Se INFRARED DETECTORS
Efficient photovoltaic detectors of infrared radiation at wavelengths up to lip. which operate at 77"K, have been fabricated from annealed Bridgman-grown Pb. Sn Se crystals.
The structure of a typical detector is shown in Fig. 1-4. The wafers are first etch-polished o
using a KOH etch. A 1000-A thick Si02 layer is then deposited on the surface and 1-mm diam-
eter circular holes are etched in the oxide by means of photo-resist techniques. The Si02 layer
is deposited on the surface to form a diffusion mask and to passivate the surface at the p-n junc-
tion. A 5- to 10-n n-type layer is then diffused in the oxide-free circular areas of the p-type
substrate in a closed-tube process using a crushed metal-rich PbSe ingot as a source. (Alter- natively, a p-type layer has been diffused in an n-type substrate using a Se-rich source.) Contact to the n-side is made by evaporating In and contact to the p-side is made by first evaporating a layer of Ag and then a layer of In. The wafer is then cut into square pieces using a wire saw and the pieces are pressure-bonded to a Cu heat sink, which has been In-plated. An In-coated wire is then pressure bonded to the small circular In contact which has been evaporated inside the 1-mm circular area, as shown in Fig. 1-4. The heat sink is mounted inside a standard micro- wave diode package, which has a window cut in the ceramic sleeve for the incoming radiation.
Figure 1-5 shows the current-voltage characteristic of a diode at 77 °K which was made from
a PbQ 9-,Sn0 064^e crvstal« The substrate in this case was p-type with a carrier concentration
of about 1. 5 x 1017cm and a mobility of 26, 000 cm /V sec.
The slope in the reverse direction corresponds to an impedance of about 5 ohms. The spec-
tral response to this diode at 77°K and at 12°K is shown in Fig. 1-6 for the zero bias condition. At 77*K the response peaks at 11 \x, near the 10.6-y. wavelength of a C02 laser. This peak, which occurs just prior to the long wavelength cut-off at each temperature, can be attributed to an in- crease in the number of electron-hole pairs reaching the junction as the radiation begins to pene- trate into the material near the bandgap absorption edge.
A large increase in the short wavelength responsivity was observed in these detectors when
the thickness of the n-type layer was reduced by etching. The responsivity near the peak of the
Section I
Fig. 1-4. Structure of a Pb^ Sn Se detector diode.
Fig. 1-5. Current-voltage characteristics of a
Pb0.936Sn0.064Se de,eC,or di0de
Section I
spectra in Fig. 1-6 is about 3 V/W at both temperatures. This value is low because the impedance
of the diode, which has an active area of about 1 mm , is only about 5 ohms. The external quantum
efficiency was estimated to be 10 percent. Since about 50 percent of the radiation is reflected at
the surface, this corresponds to a 20 percent internal quantum efficiency.
The detector noise was less than the noise of the measuring system, which yields an upper
limit of 10 V s*c. This corresponds to a minimum D* value of 3 x 10 cm/Wsec* near the
peak at 11 u. The maximum D* for a 10 percent quantum efficiency and background-limited operation with a 211 steradian field of view is about 2xio cm/Wsec1. The risetime of the detector at 77 °K was 20nsec, as measured by observing the response to the output of a pulsed GaAs diode laser. This can be accounted for by the RC constant of the device. The reproducibility of detectors made from the same wafer was essentially 100 percent in terms of cut-off wavelength. The variation in the peak responsivity was no more than 50 percent.
I. Melngailis T.C. Harman A. R. Calawa W. T. Lindley
E. InSb-GaAsP INFRARED TO VISIBLE LIGHT CONVERTER
A method is proposed and demonstrated by which infrared radiation may be efficiently con-
verted directly to visible radiation by a single solid state device. The device consists of a
capacitor-photodetector-photoemitter sandwich in which infrared radiation is incident on one face and the visible radiation is emitted from the opposite face. In order to obtain efficient conversion and allow for a tunable contrast and sensitivity, the low level current produced by
the infrared detector is integrated and stored by the capacitor and then delivered to the light
emitter in short high-current pulses. The feasibility of such a pulsed device and the efficiencies
which may be obtained have been demonstrated using an indium antimonide diode detector and a
gallium arsenide-phosphide diode emitter to convert infrared radiation of wavelengths out to
5.3 (JL into visible radiation between 0.6 and 0.7 |JL. 9 10 There have been reports on the use of photoconductors, photodiodes, and phototransis-
11 12 tors ' which utilize the photocontrolled discharge of a capacitor to integrate the detected
flux for a long time compared to the sampling time. There has also been a report of a solid state device using a Ge-GaAs heterojunction which converts 1.5-jx radiation to 0.9-p. radiation. The work we report here using InSb photodiodes greatly extends the infrared response, uses GaAsP diodes which emit in the visible, and shows that an integration technique enables one to
drive light emitters with an improved efficiency. The circuit we have used involves a capacitor in series with an InSb photodiode and a GaAsP
emitting diode. The junction capacitance of the InSb diode can be used for integration, but if this
is used in series with light emitters of comparable junction capacitance the integration advantage
can be reduced to the point of being ineffective. The structure which involves a metal-oxide-InSb
sandwich incorporating a surface depletion region as shown in Fig. 1-7 has proven to yield a rela- 14 tively high capacitance per unit area and a high detection efficiency for large area devices. The
capacitance across the oxide has exceeded the effective capacitances of the detector and emitter
diodes by an order of magnitude, thus allowing for longer integration times and overcoming limita-
tions imposed by the emitter capacitance.
Section I
PULSE V" GENERATOR
fJVVVXA/1*- VISIBLE
DEPLETION REGION
HH-W-
-w>-Q-
A drawing of a capacitor-detector-emitter sand-
wich and an equivalent schematic are shown in
Fig. 1-7. The radiation is incident on the detector
through the semi-transparent metal film and oxide
layer. During the forward biasing pulses the ca-
pacitor C is charged and the emitter E yields
pulses of visible light. Between pulses the detec-
tor D becomes reverse biased and the capacitor
discharge rate, using a low reverse resistance
emitter, is controlled by the infrared incident on
the detector. Equating the charge removed be-
tween pulses to that applied during the forward
biasing, one derives a charging current given by
-t/RC
i = c VTde
. -T/RC. (i-e ) RC
wh ere i is the detector reverse current, T , is r d the discharge interval, T is the charging pulse
Fig. 1-7. Device sandwich structure and equiva- lent schematic for producing high current driving pulses from low level infrared radiation.
length, and t is the time. This assumes the in-
tervals between pulses are sufficiently short to avoid complete discharge between pulses. If
we also assume a short charging time with T « RC, this may be written as
Thus, to have a large ratio i /i we discharge for a long time T , and charge for a short time
T . A more complete analysis leads to the result that the ratio of the change in forward cur- rent to the change in detector current has an upper limit given by the ratio of the detector leak-
age resistance to the series resistance R. The advantage of converting the low-level detector currents into higher level pulsed cur-
rents arises because most diode emitters are much more efficient at the higher current levels. To demonstrate the conversion efficiency and thresholds of detection, we connected in series an emitter diode, an InSb diode detector and a capacitor of 0.01 u.F. This value of capacitance
corresponds to that obtainable with an oxide layer on the 0.04-cm area of the detector. The emitter had an area of 0.01 cm , one-fourth the area of the detector. Using an infrared-
-4 generated continuous detector signal of 10u.A to directly drive the emitter, an output of 4 X 10
foot-Lamberts (1.4 x 10 candles /cm ) could be obtained; however, using a 100-u.sec integra-
tion time and 0.1-u.sec current pulses, the 10-u.A infrared signals were converted to 10-mA
driving pulses yielding peak light pulses of 50 fL with an average luminescence of 5 x 10~ fL, an increase of two orders of magnitude. This, accounting for the 50 percent quantum efficiency
of the detector, corresponds to the conversion of an incident 100uW/cm of 5.3-u. radiation into a signal visible with the unaided eye. The overall quantum efficiency of infrared photons to
-4 visible photons was about 10 and was principally limited by the peak efficiency of the emitter.
10
Section I
In summary, although we have not used the most efficient light emitters, we have shown that
the integration technique can be extended to 5.3 p. and used for driving emitters which are rela-
tively efficient only at high current densities. We have shown that it is possible to utilize the
large area MOS InSb diodes which incorporate a large series capacitance. Our initial attempts have yielded direct conversion of an infrared level of 100|i.W/cm into light visible to the unaided
eye. Much lower infrared levels could be seen by using a visible light intensifier in conjunction
with the infrared-to-visible converter. A more complete discussion of the advantages and limi-
tations of using the integration technique to obtain higher emitter efficiencies and to adjust for
improved contrast and sensitivity will be given in a future report.
R. J. Phelan, Jr.
F. ELECTROLUMINESCENCE IN CdS
Electroluminescence from CdS vapor-grown platelets has been observed at 4.2°K and 77°K. The high purity platelets had zero-bias resistivities greater than 10 ohm-cm. Indium contacts were usually bonded to opposite faces of the 10- to 20-^ thick samples by heating at approximately
250°C for five seconds in an H? atmosphere. Indium contacts have also been applied by pres- -4-3 2 sure bonding at room temperature. Contact areas were generally between 10 and 10 cm .
The current-voltage characteristics of the devices are as expected for space-charge-limited
flow of electrons between closely spaced, planar electrodes. Current densities as high as 4 -2 -2 2 X 10 A cm under pulsed conditions and 100 A cm DC have been obtained.
At low temperatures, luminescence at the contact edges became evident at about 10 Acm"
DC. The light intensity increased rapidly with current, varying as I ' over three orders of
magnitude in a typical device at 77°K. The external quantum efficiency was between 10* and
10~ at l,000Acm~ . Electroluminescence spectra for one sample at 4.2°K and 77°K are shown in Fig. 1-8. Relative intensities of the different peaks varied markedly from sample to sample.
The highest energy peak at 4.2°K is probably the well known I. line. As has been noted by 17 others, this line is extinguished when the temperature is raised to 77°K. The remaining peaks
have not been previously classified. The 2.507eV and 2.469 eV peaks (4.2°K) are separated by the LO phonon energy and, in addition, exhibit the same small shift in energy with temperature.
This suggests the lower of these two peaks is a "phonon replica" of the upper.
At present, there is no adequate explanation for the electroluminescence. One speculation
is that it results from ionization of electron-hole pairs by hot electrons. The average electric
fields are 10 V cm" or greater when luminescence is observed. Work will be continued with
the aim of clarifying and explaining the effect. j p Butler
G. LASER EMISSION FROM ELECTRON-BEAM-EXCITED ZnTe
Electron-beam-pumped laser emission from ZnTe has recently been reported in the litera- 18 ture. We have independently produced ZnTe lasers by electron beam excitation and have ob-
tained at liquid helium temperature 90-W peak output power with 8 percent overall power effi- o
ciency at 5280 A. Laser action at liquid nitrogen temperature with an emission wavelength of o
5310 A was also observed at somewhat reduced levels of output power and efficiency.
* Supplied by D. C. Reynolds of the Aerospace Research Laboratories.
11
Section I
PHOTON ENERGY («V)
5IOO SOOO
WAVELENGTH (A)
-2 Fig. 1-8. Electroluminescence spectra. Current density is 80 A cm at 4.2°K and 75 A cm"2 at 77°K.
The lasers were made from 10- to 100-ohm-cm p-type ZnTe crystals grown both by closed
tube vapor transport using stoichiometric ZnTe powder, and by synthesis from the elements using a Bridgman technique with a Te-rich charge* Lasers made from crystals grown by the latter method yielded higher output power and efficiency, but the number of crystals of either
type examined was insufficient to establish a meaningful pattern. The crystals were ground
and chemically polished into (110) oriented plates 50 u thick from which the lasers were then fabricated. A pair of parallel (110) cleaved faces normal to the polished (110) surface formed
the 0.5-mm wide Fabry-Perot resonator. One of these cavity faces was coated with a reflec- tive layer of evaporated aluminum. The electron beam was pulsed with 50-nsec pulses at a
rate of 60/sec. A typical set of emission spectra at liquid helium temperature for various levels of excita-
tion is presented in Fig. 1-9. Note the scale factors giving the relative emission intensities for
the curves. At the lowest beam current the spontaneous spectrum is dominated by a sharp line o 19
at 5218 A which is believed to be due to the radiative recombination of the free exciton. The
weak line at 5276 A most likely results from the recombination of this free exciton with simul- 19 ° taneous emission of a 26-meV LO phonon. In the region between 5220 and 5260 A there are
many closely spaced emission lines which, although weak at low excitation levels, increase in intensity with increasing beam current much more rapidly than does the free exciton line. As
noted by Gross et al., the ZnTe emission bears a strong resemblance to that of CdS, and
hence by analogy these latter lines could well be due to recombination of excitons bound to
•The vapor grown crystals were grown by C. W. Iseler and the Bridgman crystals by J. M. Steininger.
12
Section I
1.0 3-SS-59l9|
08 / V6* BEAM
CURRENT SCALE
FACTOR 0.6 (o) 20pA
(b) 50pA 1
3
• 0.4 r ltA\ lc) 200MA 15 3
o 0.2
a o
\0) J^A ^v i i i i i i * 0
BEAM CURRENT
SCALE FACTOR
me WAVELENGTH (A)
Fig. I-9. Emission spectra of electron-beam-excited ZnTe below [(a) - (e)] and above |(f)] the laser threshold. Beam voltage is 50 kV.
various impurity and/or defect centers. With
continued increase in current these lines, in
addition to growing rapidly in intensity, appear
to broaden until they merge into one broad spon-
taneous "line" which progressively shifts toward
longer wavelengths, presumably due to sample
heating. When the laser threshold is exceeded,
one or more very intense laser lines appear suddenly on the low-energy side of the spectral peak, as in curve (f) of Fig. 1-9. As many as three laser lines with slightly different thresh- olds have been observed simultaneously in the
o
range from 5260 to 5280 A. When examined
with high spectral resolution each of these lines
exhibits the familiar Fabry-Perot cavity mode
structure with a mode spacing which corre-
sponds to a value for (n - X dn/dX) of about 12. r o o The minimum electron energy for which
lasing could be obtained was 48keV. Over the energy range 48 to 60keV, the corresponding threshold beam current density for the best sample dropped rapidly from 13 to 4 A/cm at
liquid nitrogen temperature and from 6 to 2 1.5A/cm at liquid helium temperature. Simul-
taneously, the output power and power efficiency
increased strongly, reaching values with 60keV excitation of 25 W and 2 percent, respectively,
at liquid nitrogen temperature and 90 W and 8 percent, respectively, at liquid helium tempera-
ture. Taking into account the energy loss due to backscattering of the incident electrons ' 22 and due to absorption in the evaporated aluminum reflector, one obtains values for the internal
power efficiency of about 3 percent and 11 percent at the two temperatures. The very high values of the laser excitation threshold in ZnTe compared with those observed
in similarly excited CdS (Ref. 23) are undoubtedly due in large part to the lower quality of the
present bulk ZnTe crystals. In addition, the chemically polished surface upon which the beam
impinged was not as flat or smooth as the as-grown platelet faces of the CdS. Improvements in material technology and polishing techniques will hopefully result in a considerable reduction of
the excitation threshold for laser emission in ZnTe. C. E. Hurwitz
13
Section
REFERENCES
1. Solid State Research Report, Lincoln Laboratory, M.I.T. (1967:1), pp. 6-8, DDC 651065.
2. R. F. Brebrick and R.S. Allgaier, J. Chem. Phys. 32, 1826 (1960).
3. N. Ohashi and K. Igaki, Trans. Japan Inst. Metals 5, 94 (1964); H. Gobrecht and A. Richter, J. Phys. Chem. Solids 26, 1889 (1965).
4. R. F. Brebrick and E. Gubner, J. Chem. Phys. 36, 170 (1962).
5. W. Albers, C. Haas and H.J. Vink, Philips Res. Repts. 18, 372 (1963).
6. J.O. Dimmock, I. Melngailis and A.J. Strauss, Phys. Rev. Letters 16, 1193 (1966).
7. I. Melngailis and A.R. Calawa, Appl. Phys. Letters 9, 304 (1966).
8. J.F.Butler, A.R. Calawa, and T.C. Harman, Appl. Phys. Letters 9, 427(1966).
9. P.K. Weimer, G. Sadisiv, H. Borkan, L. Meray-Horvath, J. Meyer, Jr. and F.V. Shallcross, International Solid-State Circuits Conference, Philadelphia, Pa., Feb. 10, 1966.
10. G. P. Weckler, International Electron Devices Meeting, Washington, D.C., Oct. 20-22, 1965.
11. G.P. Weckler, International Electron Devices Meeting, Washington, D.C., Oct. 26-28, 1966.
12. G. Strull and D.H. McCann, International Electron Devices Meeting, Washington, D.C., Oct. 26-28, 1966.
13. P.W. Kruse, F.C. Pribble, and R.G. Schulze, Bull. Am. Phys. Soc. 12, 275 (1967).
14. R.J. Phelan, Jr. and J.O. Dimmock, Appl. Phys. Letters 10, 55 (1967).
15. M.A. Lamport, Proc. I.R.E. 50, 1781(1962).
16. D.G. Thomas and J.J. Hopfield, Phys. Rev. 128, 2135 (1962).
17. C.E. Bleil and I. Broser, Physics of Semiconductors (Dunod, Paris, 1964), p. 897.
18. A.N. Vlasov, G.S. Kozina, andO.B. Fyodorova, Zh. Eksp. Teor. Fiz. 52, 434 (1967).
19. E.F. Gross, L.G. Suslina, and A.l. Livshits, Fiz. Tverd. Tela 5, 801(1963).
20. E.J. Sternglass, Phys. Rev. 95, 345 (1954).
21. J.E. Holliday and E.J. Sternglass, J. Appl. Phys. 28, 1189(1957).
22. L. Holland, Vacuum Deposition of Thin Films (Wiley and Sons, Inc., New York, 1958), p. 322.
23. C.E. Hurwitz, Appl, Phys. Letters 9, 420 (1966).
14
D. OPTICAL TECHNIQUES AND DEVICES
A. C02 LASER RADAR SYSTEM
A continuous-wave CCX laser has been adapted for use in an exploratory Doppler-type laser
radar system operating at a wavelength of 10.6 |JL. This system is being used to evaluate compo-
nents and techniques for applying infrared laser devices to the problems of precision velocity and
position determination of moving objects. Presently, it is capable of measuring radial velocities
and automatic tracking of moving objects such as automobiles and pedestrians. Modifications on the system now in progress are expected to allow tracking of low-flying aircraft in the vicinity of the Laboratory; the system has been installed in a temporary shelter on the roof of one of the
Laboratory buildings. The laser is kept stationary, with beam pointing accomplished by a plane
mirror on a small radar-type pointing mount located on the roof of the shelter. In order that
orthogonal motions of the mount produce orthogonal motions of the beam for any position of the mount, it is necessary that the laser beam be brought onto the mirror along the mount's azimuth axis. This requirement was met by inverting the pointing mount on the roof above the laser beam and bringing the beam vertically upward onto the mirror. The principal disadvantage of this
method arises from obscuration of the zenith; however, this is not a serious limitation since the azimuth-elevation mounts are not useful close to zenith.
The radar system is based on heterodyne
detection of a reflected laser beam which has
been shifted in frequency by a moving object in
proportion to the relative velocity of the object. The frequency-shifted return beam is combined with an upshifted sample of the laser output on a fast square-law detector. The output of the
detector then includes a signal, occurring at the difference frequency of the two mixed beams, which is also proportional to the velocity of the
moving object. Doppler frequency shifts for reflected radiation are given by Af = 2(v/\),
which is equivalent to about 85 kHz/mile/hour.
Thus, moving automobiles and low-flying air-
craft are observed at frequencies below 20 MHz.
Figure II-1 is a drawing of the radar system
layout showing the laser source, interferome- ter, beam expander, and pointing mount loca-
tions.
Copper-activated germanium photoconduc- Fig#IM. Laser radar system. Principal components tors, produced at Lincoln Laboratory, have are shown in their relative positions.
15
Section II
Fig. 11-2. Return signal from moving automobile displayed on spectrum analyzer. Entire trace corresponds to frequency shifts of 1 MHz. Signal is seen near 100 kHz. At left is zero frequency response of analyzer.
sufficiently fast response for use in this frequency region. Teich, et al.,* demonstrated that
these photoconductors are capable of heterodyne detection of 10-\x radiation close to the theoret- ical limit of (Zhv/r\) B when operated at liquid helium temperature.
The pointing mount has been provided with three modes of beam direction: a position servo
loop for fixed directions, a "joystick" rate control for manual tracking, and an auto-track control
loop using detected laser return signals. A closed-circuit vidicon camera has been boresighted
to the laser beam axis in order to facilitate target acquisition and manual tracking. At present,
the auto-track mode is an elementary form based on detection of broad-band return signals. This
method will soon be replaced by a narrow-band variable frequency method which will permit track-
ing on smaller signals than may be used with the broad-band method. Doppler return signals have
been observed from objects up to 2 miles from the Laboratory. An example of the return signal
from a slow-moving automobile displayed on a spectrum analyzer is shown in Fig. II-2. The ra-
dial velocity is approximately l£ miles per hour. H. A. Bostick
B. SEALED C02 LASER TUBE WITH GREATER THAN A 700-HOUR LIFE
Work has continued with a CO laser tube that is similar to the tube discussed in the last t Solid State Research Report. The difference between this tube and the tube reported on was in
the choice of Ni rather than Kov^.r for the cathode metal. Adsorption at the cathode of the carbon-
oxygen compounds made the choice of metal type a critical one, and Ni was chosen because its
reactions with these compounds was likely to be reversible. Figure II-3 shows the concentrations,
indicated as pressures, of the molecules that were in the tube versus operating time. The con-
centrations were determined by mass analyzing gas samples taken from the operating tube. Only CO and O- were formed in concentrations comparable to the concentration of the initial fill gases
of 2 torrs C02, 2 torrs N,, and 7 torrs He. No nitrogen-oxygen compounds seemed to be found other than at impurity concentrations. After the initial adsorption within 335 hours of operation, the gases were easily desorbed when the cathode temperature was raised to 300 °C. When the
C02 was adsorbed to a pressure of 0.2 torr the laser output power dropped to zero; but when
the desorption was complete with a redemption of 80 percent of the CO^, the output power was
*M.C. Teich, R.J. Keyes and R.H. Kingston. Appl. Phys. Letters 9, 357 (1966).
t Solid Slate Research Report, Lincoln Laboratory, M.I.T. (1967:1). DDC 651065.
lb
Section II
J > IJ-SS-SIJSl |
\
~ 1 i j e s - t \ w - ->o
\ M \
\ a
a^--"' \S •\
*"'" a
\ \ \ \
* \ \ \ 6 \ \ \ k co2 \co
1 > LH^O L_ \ ' \
T-^^4) 1 100 200
TUBE OPERATION (hour»)
Fig. 11-3. Partial pressures of CO, O«, CO«, N? and He in laser tube shown as a function of operating time.
resumed at its initial value. The adsorption-desorption process was cycled several times up
to 709 hours continuous operation with a resumption of normal laser power output, and at this time 80 percent of the original fill of C02 was still being reclaimed. At the 709-hour point, the
pressures of the major constituent gases were established as 1.7 torrs C02, i.2 torrs N-, 0.25 torr CO, 7 torrs He, 0.03 torr 02, and the major impurity of H2 at 0.48 torr. Inad-
vertently, water was dropped on one Brewster NaCl window destroying its optical flatness and
the tube ceased lasing at this time.
It seems that the C02> CO, and 02 interacted with the Ni and NiO and carbon surfaces
forming a carbonate, CO^ complex, on the surfaces which was desorbed as C02 and CO when
the cathode region was heated. The carbon was most likely formed in the sputtering process while the NiO was formed with the free oxygen or an oxygen complex on the sputtered Ni film.
The specific power generation of 0.15 W/cm for this 10 mm bore laser was not too far different from the 0.25 W/cm generated in a flowing gas tube of the same diameter, and the difference
could be due to the higher concentration of CO and 02 and the higher gas temperature in the
sealed tube versus the flowing tube. R j raruone
C. PHOTOVOLTAIC HETERODYNE DETECTION IN Pbt _vSnSe
A Pb. Sn Se photovoltaic detector has been employed to observe heterodyne detection, using
scattered radiation from a CO, laser as a signal at 10.6 u.m. The observed minimum detectable -14 power, at a frequency of 110 kHz and in a bandwidth of 65 kHz, was 1.8 x 10 watt, which is to
-14 be compared with the theoretical value of 1.7 x 10 watt. At these low frequencies, present
17
Section II
photovoltaic devices perform as well as Ge.Cu photoconductive detectors, and have the distinct
advantage of operation at 77 °K rather than at 4 °K. Work is now being directed toward operation at higher frequencies and in wider bandwidths.
M. C. Teich
D. FARADAY ISOLATOR
Faraday rotation and absorption have been measured at 10.6 fim in InSb in order to construct
an isolator for the CO., laser. Sufficient rotation to obtain isolation (45°) has been obtained in 17 3 a 0.5-mm thick sample with a carrier concentration of 2.3 X 10 cm , and a mobility of
45,000 cm /V sec at 5200 gauss. The sample absorbs 10 percent of the beam energy. The
isolation capability of the device will be limited by the uniformity of the field and the perform-
ance of the polarizer. The power handling capability will be limited by the thermal conductivity
of the material. Work is in progress to determine these limitations.
Jane H. Dennis
tfl
HI. MATERIALS RESEARCH
A. CRYSTAL GROWTH
1. MnBr2
Crystals of MnBr, have been grown from the vapor phase and from the melt by a method 1 similar to the one used to obtain Mnl2 crystals. Helium was saturated with bromine vapor by
bubbling it through liquid bromine at 0°C, and then passed over a quartz boat containing man-
ganese powder heated to 850°C at the entrance to a muffle furnace with a decreasing temperature
gradient. Under these conditions the Br2 and Mn reacted to form MnBr2 vapor. Most of this
vapor condensed at temperatures above about 700°C as a liquid which solidified on cooling to form large transparent grains about 3 mm thick. The balance of the MnBr2 vapor condensed in
a cooler region of the furnace as single crystal platelets of which the largest was about 2 X 2 x
0.005 cm. It was found that MnBr2 is considerably less hygroscopic than Mnl2, and can be ex-
posed to the atmosphere for several days before deteriorating. T. B. Reed W. J. LaFleur
2. EuO
A large-grained ingot of EuO, an infrared-transparent ferromagnetic material, has been ob- tained by a modification of the method described by Guerci and Shafer. The ingot was prepared
from a nonstoichiometric melt in a molybdenum crucible 7 cm long, with an outside diameter of 3 cm and a maximum inside diameter of 1 cm. The inner chamber was tapered to permit easy removal of the ingot. The crucible was loaded with 3.0g of Eu metal and 3.5g of Eu20-, corre-
sponding to a composition of Eu. 330, and then sealed to prevent vaporization losses by means
of a deformable molybdenum washer held in place by a threaded cap. The charge was heated in
a resistance furnace to 2000°C for 1 h, cooled to 1100° at the rate of 9°C/h, and then cooled
rapidly to room temperature. The dark-red ingot formed was 1.5 cm long and contained single
crystal grains up to 4mm across. A sample from the ingot gave a Debye-Scherrer pattern con- taining only lines of the fee EuO phase. The lattice parameter a determined from this pattern is 5.1438 ± 0.004Ä, compared with literature values of 5.1451 ± 0.0003Ä (Ref. 2) and 5.1439A
(Ref- 3>- T. B. Reed R. E. Fahey E. R. Pollard
B. KINETICS OF ELECTRON TRANSFER BETWEEN CONDUCTION BAND AND SULFUR DONORS IN GaSb
At atmospheric pressure, the lowest sulfur donor level in GaSb lies about 0.075 eV below
the lowest conduction band, which has r symmetry. Measurements of resistivity as a function 4 1
of pressure have shown that this donor level is associated primarily with the X. conduction
band minima, which lie at an energy estimated to be about 0.3 to 0.4 eV above the T. minimum.
19
Section III
At temperatures below about 100°K, nonequilibrium effects were observed in resistivity
and Hall coefficient measurements on S-doped GaSb. These observations indicated that the
rate at which electrons are transferred from the conduction band to the sulfur donor levels
decreases so rapidly with decreasing temperature that it becomes readily observable in the vi-
cinity of 100°K. A series of isothermal measurements has been made to determine the tem-
perature dependence of the transfer rate. In each experiment the sample was first cooled from
room temperature by plunging it into a mixture of liquid oxygen and liquid nitrogen in the cor-
rect proportions to give the desired temperature, and the resistivity was then measured as a
function of time. When the sample reaches the bath temperature, its resistivity has increased
by about two orders of magnitude due to the deionization of the sulfur donors, but the concen-
tration of electrons in the conduction band still exceeds the equilibrium concentration. There-
fore the resistivity slowly increases to its equilibrium value as electrons fall from the con-
duction'band into empty donor levels.
The results for a typical experiment, conducted at 90.2°K, are shown in Fig. III-l. In order
to facilitate analysis, the data are presented in terms of conductivity rather than resistivity.
In particular, the difference between the conductivity at a given time and the final, equilibrium
conductivity is plotted logarithmically versus time. Since it was found that the electron mobil-
ity at constant temperature does not change significantly with carrier concentration, the conduc-
tivity difference A
Section III
satisfactorily as the sum of two straight lines corresponding to two different time constants de-
fined by the expression Aa = A expf-t/r^ + B exp(-tA2). The temperature dependence of the two time constants is shown in Fig. III-2, where the
logarithm of T is plotted against reciprocal absolute temperature. The slower time constant
increases from 5 min. at 90°K to 1300min. at 77°K, and the faster time constant increases from 1.6 to 230 min. over the same temperature range. Both constants vary exponentially with recip-
rocal temperature, since the data are well represented by two straight lines. The activation
energies calculated from the slopes of these lines are 0.255 eV for the slower time constant and
0.23 eV for the faster one. These energies may be regarded as the same within the limits of
experimental error. Nonequilibrium effects like those described have not been reported previously for GaSb nor
any other pure III-V compound, although quite similar observations have recently been made on
S-doped GaAs, P alloys. At low temperatures, the conduction electrons in the GaSb samples
are almost entirely in the I\ conduction band. Since the lowest sulfur donor levels are asso- ciated primarily with the X. conduction band, it seems possible that the cross section for scat-
tering from a I\ state to an X4 state might be small enough to account for the observed transfer
rates. No attempt has been made to calculate this cross section theoretically. It should also be
noted that an explanation for the two different time constants characterizing the decay of conduc-
tion electrons is not obvious on the basis of this model. G. W. Iseler A. J. Strauss
|l-M-iT?0ll)l
C. POLYMORPHISM IN Ag2Te AT HIGH PRESSURES AND TEMPERATURES
Previous electrical resistivity and x-ray diffraction studies on Ag2Te revealed the existence at room temperature of two high-pressure phases, one stable between about 22 and 25 kbars and
the other stable above 25 kbars (Ref. 7). In order to
obtain further data on the boundaries of these phases, additional resistivity measurements have been made as a function of temperature and hydrostatic pressure.
The results obtained have been used to construct the
partial P-T diagram for Ag2Te shown in Fig. III-3,
where the high-pressure phases are designated as
Ag2Te-II and Ag2Te-III. Since the transition points shown inFig.III-3 were
located by changes in the slope of resistivity curves measured as a function of temperature (isobars) or pressure (isotherms), the accuracy of these points de-
pends on the abruptness of the changes in slope. For
the I ■* II and II ■* III transitions, the changes are quite sharp at room temperature but become increasingly
gradual above 50 °C. Therefore the I-II and II-III phase
boundaries are shown as dashed lines in Fig. III-3.
There is some indication that these two boundaries in-
tersect at some temperature below the boundary of the
f.c.c. phase.
m TETRAGONAL
20
PRESSURE (kbors)
Fig. III—3. Pressure-temperature diagram for Ag2Te.
21
Section III
The crystal structure of Ag-,Te-III has been determined from x-ray diffraction patterns ob- 8 tained with a new diamond-squeezer high-pressure unit designed by Bassett. This apparatus
has two principal advantages: the x-ray beam is finely eollimated and precisely centered on the
face of the piston diamond, and the sample while under pressure can be observed through the an-
vil diamond with a metallographic microscope. These features are important because in this
type of equipment the pressure is highest at the center of the piston face and decreases markedly
toward the periphery. Consequently, in many cases more than one phase is present. Since var- ious phases differ in their reflectivity, both for polarized and unpolarized light, with the Bassett
camera their locations can be determined visually. Therefore it is possible to see whether the
pressure can be adjusted so that the entire area of the x-ray beam is intercepted by a single
phase, as required for an unambiguous diffraction pattern. This could be done for Ag-Te-III, as shown by the high-pressure photomicrograph of Fig. III-4. The dark area in the center is
AgpTe-III, the adjacent lighter area is Ag2Te-II, and the dark ring at the periphery of the piston
is the atmospheric pressure monoclinic phase.
Fig. II1-4. Photomicrograph of AgoTe sample under pressure in Bassett x-ray camera, snowing concen- tric areas of Ag2Te-lll (center,) Ag2Te-ll/ and Ag2Te-l (outside).
The x-ray pattern for Ag^Te-III has been indexed on a tetragonal cell with a = 8.80Ä,
c = 6.20A, and c/a = 0.71. The hki values assigned to the diffraction lines, together with the observed and calculated d-spacings for this indexing, are given in Table III-l. The calculated density at pressure is 9.5g/cm , assuming 8 molecules per unit cell. This corresponds to a density increase of 13 percent with respect to the monoclinic phase at one atmosphere.
22
Section III
TABLE lll-l
INDEXING OF X-RAY PATTERN FOR HIGH-PRESSURE Ag2Te-lll (TETRAGONAL)
hk*
d-spacing (A)
Intensity Observed Calculated
221 3.31 3.32 1
300 2.93 2.93 10
221,112 2.74 2.77 4
311,202 2.54 2.54 8
320 2.40 2.44 2
321 2.25 2.27 0.5
400 2.20 2.20 4
302 2.13 2.13 1
312,330 2.08 2.07 5
331,103 1.98 1.97 1
203 1.88 1.87 1
402 1.80 1.79
Section
D. EFFECT OF HIGH PRESSURE ON OXIDES WITH DEFECT-ROCKSALT STRUCTURES
The monoxides of Ti, V, and Nb are compounds which contain unusually large concentra-
tions of lattice vacancies, located at both metal and oxygen sites. For the Ti and V oxides, the
range of homogeneity is quite wide, with x between 0.8 and 1.25 for TiO . Up to 15 percent of
all sites are vacant. When prepared by the usual methods, these two compounds have the rock-
salt (Bl) structure, with the vacancies randomly distributed through the lattice. (Ordered struc-
tures can also be prepared by prolonged annealing at sufficiently low temperatures.) For NbO , the range of homogeneity is much narrower (0.98 < x < 1.03). In this case, 25 percent of the
sites are vacant, and the vacancies are ordered to give a structure which is different from Bl,
but related to it.
A study is being made to determine whether the concentration of vacancies in the three
monoxides can be significantly reduced by the application of high pressures. Most of the exper-
iments so far have been made on polycrystalline TiO ingots obtained by quench casting of melts
prepared by arc melting mixtures of crystal bar Ti and high purity TiO- in appropriate propor-
tions. These samples are significantly higher in purity than those we have made by other meth-
ods. The stoichiometry is determined gravimetrically by measuring the weight gain on combus-
tion to Ti02. The x-ray density is calculated from the cubic lattice parameter a determined from the diffraction pattern obtained with a Debye-Scherrer camera. The a values are accu-
o o ° rate to ±0.001 A for good films and to ±0.003 A for poorer ones. The measured density is found with a micropycnometer, using toluene as the pycnometric fluid. The concentrations of Ti and O vacancies are then calculated by comparing the x-ray and measured densities according to the method described in the literature. '
In a typical experiment, the TiO sample is first annealed for ~ 20 hours at 1500°C in vac-
uum or under argon at about 500 torrs. After the x-ray and pycnometric measurements are
made, a portion of the sample is annealed for 1 to 3 hours at about 50 kbars and 1100 to 1300°C,
and the densities are again measured. Representative results for samples with x between 0.87
and 1.06 are given in Table III-2. In every case annealing under pressure caused a decrease of
at least 15 percent in the calculated total vacancy concentration. The magnitude of the decrease appears to depend more on the purity and stoichiometry of the material than on the pressure, temperature, and time of anneal. Application of pressure caused an increase in a which re- sulted in a decrease in the x-ray density although the measured density increased. The increase
in a is due to the decrease in vacancy concentration, since the effect of a large concentration
of randomly distributed vacancies is to decrease the apparent average bond length and therefore the size of the unit cell.
The data in Table III-2 also show that the reduction in vacancy concentration was accom-
panied by an increase in the superconducting transition temperature T for TiO . For samples 12 ex
prepared at atmospheric pressure it has been reported that T varies strongly with x, reach-
ing a maximum of about 1°K for x = 1.0. Since the lower limit of measurement in the present
experiments is about 1.2°K, no superconducting transition could be observed in vacuum-annealed samples. After annealing under pressure, most of the samples had transitions between 1.3 and
1.8°K, although in some cases a well-defined Tc could not be determined because the transitions
24
Section
TABLE II1—2
EFFECT OF PRESSURE ON TiO X
X Treatment
a o
(±0.001) (A)
X-ray Density
(g/cm )
Pycnometric Density
(g/cm )
Fraction of Sites Occupied
Total Vacancies (percent)
Decrease in Vacancies
(percent)
T
(°K) Ti O
0.872 A* 4.190 5.580 5.043 0.904 0.788 15.4 17
Section III
pressure. Over the range from atmospheric pressure to 6 kbars. the limit of the measurements,
Tc was found to decrease linearly with increasing pressure at the rate of 0.63°K/kbar. This re-
duction in Tc supports the hypothesis13 that the magnetic properties of SrRu03 are primarily due
to band effects rather than to localized moments. J. A. Kafalas N. Menyuk
K. Dwight J. B. Goodenough
F. ELECTRON DENSITY DIFFERENCE MAP AS FURTHER EVIDENCE FOR TRAP-MEDIATED Pb-Pb BOND IN PbRu03
The existence of a trap-mediated Pb-Pb bond in PbRu03, a compound with pyrochlore struc- ture, has been proposed on the basis of a structure refinement which indicated that oxygen va-
cancies at the 8(b) sites in the lattice act as traps for electrons transferred from the four Pb ions which are arranged tetrahedrally around each of these sites.14 Evidence supporting this model
was obtained by comparing the ionic radius of Pb in this compound with its radii in other mate- rials with pyrochlore structure.
In order to obtain more direct evidence concerning the proposed model, an electron density difference map has been calculated for PbRu03. To obtain this map, a Fortran computer pro-
gram was written to generate electron density maps from structure factors for x-ray scattering. This program was used to obtain theoretical electron distributions in PbRu03 from the form
factors for O2' (Ref. 15), Pb2+, and Ru4+ (Ref. 16) and the known positions of Pb, Ru, and O in
the compound. Since the atom positions and space-group were known, the values of the phase
angle a could be obtained from the same theoretical form factors. (Although the structure is
Fig.lll-5. Electron density difference map for PbRuOß, for a section at z = 0.125 par- allel to the (001) planes.
Id
Section III
centrosymmetric, values of a as different from 0 and IT as 0.2 and 2.8, respectively, were ob-
tained because of the contribution of the imaginary part of the correction for anomalous disper-
sion. If this term had been neglected, the calculations would not have been of sufficient accuracy.)
The same Fortran program was used to compute experimental electron distributions from the
structure factors calculated from integrated x-ray intensity data for PbRuCL. Wherever reflec-
tions overlapped, the contribution of the individual structure factors was deduced from the theo-
retical ratios. Including all the positive and negative permutations, summations were made for
1242 (hkf) combinations.
In order to investigate the whole cell, electron density maps were obtained for three two- dimensional sections, parallel to the (001) plane, located at z - 0, 0.125, and 0.25. Analysis of
these maps showed that 2.0 ± 0.3 electrons were transferred from each of the lead atoms. A Fourier difference map, showing the difference between the theoretical and experimental elec- tron distributions, was then made for the section at z = 0.125, as shown in Fig. II